Support for .mill and .mill.scala files in VSCode/Metals (1000USD Bounty)

[lihaoyi]

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From the maintainer Li Haoyi: I'm putting a 1000USD bounty on this issue, payable by bank transfer on a merged PR implementing this.

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Mill 0.12.0 is moving towards using build.mill/package.mill files rather than build.sc. Basic IntelliJ support landed in JetBrains/intellij-scala#667, so the next thing to do is get them working in VSCode/Metals. .mill and .mill.scala files already mostly work in IntelliJ/BSP, so this ticket is to get the same support for .mill and .mill.scala files in VSCode/Metals/BSP as well.

The acceptance criteria is as follows:

• Download and unpack https://github.com/com-lihaoyi/mill/releases/download/0.12.0-RC3/0.12.0-RC3-example-depth-large-10-multi-file-builds.zip
• Load the folder's Mill build into Metals
• Inside bar/qux/package.mill
  • Jumping to build.MyModule should bring you to build.mill
  • Jumping to build.MyModule should bring you to mill-scalalib_2.13-0.12.0-RC3-sources.jar/mill/scalalib/package.scala (or equivalent)
• Inside foo/package.mill
  • Jumping to build.MyModule should bring you to build.mill
  • Jumping to build.bar.qux.mymodule should bring you to bar/qux/package.mill
  • Jumping to RootModule should bring you to mill-main_2.13-0.12.0-RC3-sources.jar/mill/package.scala (or equivalent)
• Inside build.mill
  • Jumping to ScalaModule should bring you to mill-scalalib_2.13-0.12.0-RC3-sources.jar/mill/scalalib/ScalaModule.scala (or equivalent)
• Everything should still work if the .mill files are renamed to .mill.scala

For the purposes of this ticket, treating .mill files as vanilla .scala files (with the appropriate classpath and sources) is sufficient. It's not 100% precise, but is good enough for 95% of scenarios, and we can sort out the last 5% in a followup. Notably, we do not want to treat them as .sc files, since things like ScalaCLI directives and other scripty things don't work

Prior Art: scalameta/metals#6752

Dependencies

• [BSP] semanticDbData fails on meta-build #3666